[Al Skierkiewicz]

You need to know the specifics of the camera and lens you are using to be ultimately accurate. If you know the focal length of the lens and the size of the pickup device, you can make a calculation using trig, to determine the angle of the field of view. Once you have that, you need to know the distance to target or you can back into the distance by calculating a known target size as a percentage of the field of view. This might be a frustrating exercise since the pickup and focal length of these lenses is so small. That allows a lot of error to creep into the calculation.